Fuel burner control system



Dec. l5, 1942. w. l.. McGRATH 2,305,544

FUEL BURNER CONTROL SYSTEM Filed May 6, 1940 Dec. 15, 1942 Patented FUELBURNER CONTROL SYSTEM William L. McGrath, Philadelphia, Pa., assignor toMinneapolis-Honeywell Regulator Company, Minneapolis, Minn., acorporation of Delaware Application May 6, 1940, Serial No. 333,494

8 Claims.

This invention relates broadly to automatic control systems, and moreparticularly to systems for automatically controlling the starting andstopping operations of fluid fuel burners.

In simplifying and reducing the expense of fuel burner control systemsit is desirable in many instances to eliminate all electromagneticrelays. However, it has been found under these circumstances that thesystem has no protection against power failure; that is, on powerfailure the burner motor will stop delivering fuel and combustion willcease, but if power is returned again before the protective combustionresponsive device has cooled sufficiently to open its hot contacts,then, upon the resumption of power, the burner motor will be immediatelyreenergizcd without the advantage of a scavenging period.

It is therefore a prime object of this invention to provide a system ofthe above type which has protection against a momentary power failure.

A further object of the invention is to provide a control system inwhich the running circuit for the burner motor is dependent upon thepressure developed by the burner motor so that a momentary failure ofthe power driving the burner motor will result in opening the runningeircuit.

A still further object is to provide, in a system of the above type, ameans for delaying the reclosing of the starting circuit following aburner motor shut down whereby a scavenging period will be produced.

A still further object is to make the running circuit to the burnermotor dependent upon the fuel pressure developed by the burner motor sothat if the burner motor or fuel pressure should fail for any reason theburner motor will be immediately shut down and not reenergized until theelapse of the proper scavenging period.

These and other objects will readily become apparent as the followingspecification is read in the light of the accompanying drawing in whichFigure l is a diagrammatic representation of one form of my invention,and

Figure 2 is a diagrammatic representation of a modified form.

Referring now to Figure 1 of the drawing, the reference numeral IIindicates a space thermostat comprising a bimetallie element I2 andswitch arm I3. The thermostat is so arranged that on a decrease intemperature the bimetallic element I2 will move the switch arm I3 intoengagement with the stationary contact I4. If desired, a permanentmagnet I5 or some equivalent device may be used for obtaining a snap 23and 2|, respectively. A bimetallic thermostat 22 is normally latchedbeneath the free end of blade I9 thereby holding the contacts 20 and 2|in engagement. If the bimetallic element 22 is heated to a predeterminedextent by means of the electric heater 23, it will warp toward the rightand free the resilient arm I9. The arms I8 and I8 will therefore flexdownwardly until they engage the legs 24 and 25, respectively, of themanual reset member 26. The leg 25 will hold the contact 2|] out ofengagement with the contact 2| thereby maintaining the safety switch inopen circuit position. Alter the thermal element 22 has cooled, theswitch may be manually reclosed by pushing the reset member 26 upwardlyuntil the arm I9 is swung free of the thermal element 22. At this timethe thermal clement 22 is permitted to return to its normal coolposition shown in Figure 1 and when the reset member 26 is released theparts will return to the position shown in Figure 1 in which thecontacts and 2| are in engagement. It will be noted that as the switchis being manually reset the leg prevents engagement ol contact 20 withcontact 2| so that these contacts may never be manually held inengagement.

The control system is provided with the usual burner motor 28 whichdrives a fuel pump 29. The conduit 30 connects with a source of fue]supply and conducts it to the pump 29. A second conduit 3| is connectedto the pressure side of the pump 29 and communicates with a pressureresponsive bellows 32. The bellows 32 is adapted to actuate an overcenter snap switch generally indicated at 33 and comprising articulatedlinks 34 and 35 and an over center spring 36. The bellows 32 isconnected to the link 35 by a connecting means 31. When the pressurewithin the bellows 32 is at a relatively low value, the link 34 is heldadjacent the stop 38 by spring 36, but when the pump 29 is energized tobuild up the pressure for delivering the oil to the burner nozzleindicated diagrammatically at 40, the pressure within the bellows 32will increase, resulting in the switch 33 being snapped to its oppositeposition in which the link 34 engages the stationary contact 4I. Whenthe pump is stopped or the oil pressure fails for any reason, thebellows 32 will contract and the return spring 42 will return the snapswitch to its original position as shown in Figure 1.

The control system is provided with the usual combustion control deviceindicated generally at 43. This device may comprise a bimetallic elementinserted into the stack and control the switch arm 44 by means of a slipfriction connection. The switch arm 44 is adapted to engage thestationary contacts 45 and 46. When the stack thermal element is coolindicating an absence of combustion, the switch arm 44 is in engagementwith its cold contact 45. As combustion is established and the stackthermal element heats up, it operates rst to cause the switch arm 44 toengage the hot contact 46 and later to disengage the cold contact 45.When the combustion is terminated and the stack thermal element cools,it first causes the switch arm 44 to disengage the hot contact 46 andlater to engage the cold contact 45. Due to the slip friction connectionbetween the stack thermal element and the switch arm 44, the latter willengage and disengage the contacts 45 and 46 in response to changes intemperature rather than perform this function at any predeterminedtemperature value. Control devices of this type are well known to theprior art at this time and form no part of the present invention, andany device which actuates hot and cold contacts as described above inresponse to combustion may be used.

Operation Figure 1 illustrates the system with the parts in the positionwhich they will assume when the room thermostat is satisfied, therebymaintaining the switch blade I3 out of contact with the stationarycontact I4. The room thermostat has been satisfied for a sufficientlength of time to permit the stack thermal element to cause the switcharm 44 to disengage the hot contact 46 and engage the cold contact 45.Under these conditions, the room thermostat II prevents the delivery ofelectrical energy to the control system.

On a decrease in temperature in the space in which the thermostat II islocated, the bimetallic element I2 will contract and move the switch armI3 into engagement with the stationary contact I4. This will establishthe following electrical circuit: from supply conductor 50, bimetallicelement I2, switch arm I3, contact I4, conductor 5I, resilient arm I8,contacts 20 and 2I, arm I9, conductor 52, electric heater 23, conductor53, contact 45, switch arm 44, conductor 54, burner motor 28 and supplyconductor 55. This circuit will simultaneously energize the heater forthe thermal actuator 22 of the safety switch II and the burner motor 28which drives the oil pump 29. As a result of the operation of the oilpump 29, the oil pressure will be built up on the pressure side of thepump and the bellows 32 will immediately expand and cause the arm 34 toengage the stationary contact 4I. This action at the present timeestablishes no additional electrical circuit because the stationarycontact 46 which is connected to the contact 4I by means of conductor 56is not in engagement with the switch arm 44. If combustion is notestablished, the stack thermal element will remain cold and thereforethe switch arm 44 will not be moved. This means that the electric heater23 will contlnue to heat the safety switch thermal actuator 22 and aftera predetermined time the latter will warp toward the right as seen inFigure 1 and release arm I9, thereby opening the safety switch contactsand 2| as described above. This will break the circuit to the burnermotor and the oil pump will be stopped. Bellows 32 will thereforecontract and return switch 33 to open position. The condition whichprevented combustion being established must then be corrected and thesafety switch manually reset to closed position.

If the room thermostat is still calling for heat, the starting circuittraced above will be re-established upon the closing of the safetyswitch and the burner motor will again be energized to operate the oilpump, and as a result of the operation of the oil pump the bellows 32will cause the arm 34 to engage stationary contact 4I. If combustion issuccessfully established, the stack thermal element will heat and causethe switch arm 44 to engage the hot contact 36. This will establish ashunt circuit around the safety switch heater 23 from the resilient armI9 through conductor 58, links 35 and 34, contact 4I, conductor 56, andhot contact 46 to the switch arm 44. At this time therefore, the safetyswitch heater 23 will be shunted out of control and a running circuitfor the burner motor 28 is established independently of the heater 23and cold contact 45, through the pressure switch 33 and hot contact 46.Further heating of the stack thermal element will cause the switch arm44 to disengage the cold contact 45. This is the running condition ofthis system.

When the space thermostat II is satised, the bimetallic coil I2 willexpand and cause the switch arm I3 to disengage the stationary contactI4, thereby cutting olf the supply of electrical energy to the controlsystem. This will deenergize the burner motor stopping the fuel pump andthe bellows 32 will immediately contract and open the snap switch 33returning it to the position shown in Figure l. As a result of thecessation of combustion, the stack thermal element will cool,immediately disengaging the switch arm 44 from the hot contact 46, andafter a predetermined length of time sufficient to properly scavenge thecombustion chamber the switch arm 44 will reengage the cold contact 45.

If a flame failure should occur while the system is in runningcondition, the stack thermal element will cool and cause the switch arm44 to disengage the hot contact 46. This will break the running circuitto the burner motor and as the starting circuit has already been brokenat the cold contact 45 the burner motor will be deenergized. After apredetermined time interval, the switch arm 46 wil reengage the coldcontact 45, thereby reclosing the starting circuit. If combustion issuccessfully established, the system will return to its runningcondition. If not, the thermal element 22 will be heated to apredetermined temperature by the heater 23 and unlatch the safety switchI'I, thereby shutting the system down until it is manually reset.

It will be seen that when the system is in running condition, thecontinued operation of the burner motor and fuel pump is dependent uponthe maintenance of the fuel pressure by the pump to keep the bellows 32expanded. If this pressure should drop due to a power failure or afailure in the fuel supply, then the switch arm 34 will disengage thecontact 4I, breaking the running circuit to the burner motor 28. Animmediate return of power or fuel will not result in the immediatereenergization of the burner motor because once the running circuit hasbeen broken the burner motor cannot be reenergized until the originalstarting circuit has been reclosed by the combustion responsive switcharm 44 engaging its cold contact 45, and as explained above, this willnot occur until the proper scavenger pcLiod has elapsed. As the fuelpressure to which the bellows 32 responds will drop immediately, theburner motor is deenergized due to power failure. It will be seen thatthe present system protects fine.

` against a momentary power failure by preventing reenergization of theburner motor until after the combustion chamber has been scavenged.

The modified control system disclosed in Figure 2 is provided with aroom thermostat 60 and a safety switch 6I. The safety switch 6I has theusual thermal actuator 62 and electric heater 63 therefor. The roomthermostat and safety switch of Figure 2 are identical with those ofFigure i, and therefore it is not deemed necessary to describe themagain in detail.

The combustion responsive device 64 is provided with a switch arm 65connected to a thermal element through the usual slip frictionconnection. In this modification, the switch arm 65 has no cold contactbut cooperates only with the hot contac?l 66. This modification also hasa burner motor 61 and fuel pump 68. A conduit 69 connects the lowpressure side of the fuel pump 68 with a source of fuel supply and theconduits 10 and 1| connect the high pressure side of the fuel pump 68with the pressure responsive bellows 12. The conduits 10 and 1| areconnected by means of a restriction 13 which is in the form of anadjustable bleed valve 14. Due to the restriction 13, when the pump 68ls energized the bellows 12 expands slowly over a predetermined periodof time rather than expanding immediately as in the case of Figure l.The bellows 12 is connected by a connecting means 16 to a link 11 of anover center snap switch 18. The link 11 is pivotally connected to thelink 19 which is snapped back and forth by means of an over centerspring 80 as the bellows 12 expands and contracts. The link 11 isprovided with a return spring 82 to return the switch upon contractionof the bellows 12. When the bellows 12 is contracted, the arm 19 engagesthe stationary contact 83, and when the bellows expands the switch issnapped to the position where the arm 19 engages stop 84.

The bellows 12 operates a second switch arm 85 by means of a slipfriction connection 86 diagrammatically illustrated in the drawing. Theswitch arm 85 is adapted to oscillate between the stationary contact 88and a stop 89. The travel of the arm 85 is adjusted to be very slight,and due to the slip friction connection the arm 85 is adapted to engageand disengage the stationary contact 88 immediately the bellows startsto move, whereas the over center switch 18 is actuated only after thebellows has contracted or expanded substantially its full amount.Therefore the switch arm 85 will always engage or disengage itsstationary contact 88 before the switch arm 19 will disengage or engageits contact 83. It will be apparent that by adjusting the restriction 13by means of the valve 14 the time required for the bellows 12 to expandand contract its full amount may be varied. Therefore the time intervalbetween energization of the burner motor 61 and the opening of theswitch 18 may be increased by further restricting the opening at 13 andmay be decreased by enlarging the opening. Inasmuch as the arm 85 has avery slight travel and is operated through a slip friction connection,the time necessary to operate the switch 85-88 will not be materiallyaffected by adjusting the valve 14.

The system is shown in Figure 2 with the parts in the position whichthey assume when the thermostat 60 is satisfied and has been satisfiedfor a sufficient length of time to permit the bellows 12 to contract itsfull amount and for the switch arm 65 to disengage the hot contact 66and assume its cold position. On a decrease in room temperature the roomthermostat 60 will cool and Search Room establish a circuit from supplyconductor 90 through the room thermostat 60, conductor 9|, safety switch6l, conductors 92 and 93, heater 63, conductor 94, stationary contact83, links 19 and 11, conductors 95 and 96, burner motor 61, and back tothe other supply conductor 91. This circuit Will simultaneously energizethe burner motor 61 and the electric heater 63 for the safety switchthermal element 62. The energization of the burner motor operates thepump 68 and the bellows 12 will immediately start to expand and at rstcause the switch arm to engage the stationary contact 88. This will notestablish an additional circuit at the present time due to the contactswitch arm 65 is not in engagement with the hot switch contact 66.

In order for this system to be safe, the time necessary for the bellows12 to expand sufficiently to operate the snap switch 18 must be greaterthan the time necessary for the heater 63 to heat the thermal element 62sufficiently to open the safety switch 6l. Therefore, if combustion isnot established when the above starting circuit is closed, the thermalelement 62 will open the safety switch and lock the system out beforethe bellows 12 causes the arm 19 to disengage the stationary contact 83.The opening of the safety switch 6l will cut off the supply of power tothe burner motor 61 and the bellows 12 will contract to its originalposition to cause the switch arm 85 to separate from the stationarycontact 88.

However, after combustion is successfully established, the combustionresponsive device will cause the switch arm 65 to engage the stationarycontact 66 and establish a holding circuit for the burner motor throughthe safety switch 6l, conductors 92 and 98, contact 66, switch arm 65,conductor 99, contact 88, switch arm 85, conductors |00 and 96 to theburner motor 61. This circuit shunts the safety switch heater 63 andestablishes a running circuit for the burner motor 61 independently ofthe switch 19-83 which is subsequently opened by means of the furtherexpansion of bellows 12.

When the room thermostat 60 is satisfied, it opens the circuit betweenthe conductors and 9| and the burner motor 61 is deenergized and theparts returned to the position shown in Figure 2. It will be noted thatthe original energizing circuit for the burner motor cannot be recloseduntil the bellows 12 has contracted sufficiently to close the snapswitch 18. Therefore, the scavenging period for the system is timed bymeans of the bellows 12 and this timing, as explained above, may readilybe adjusted by the valve 14. If the system is in running condition and apower failure should occur, the burner motor 61 will stop and thebellows 12 will immediately start to contract. As soon as this takesplace, the bellows through its slip friction connection 86 willimmediately cause the switch arm 85 to separate from the stationarycontact 88, thereby breaking the running circuit to the burner motor 61.If power is now resumed the burner motor will not be reenergized untilthe bellows 12 has contracted sufficiently to reclose the snap switch18.

On a flame failure it will be apparent that the stack thermal elementwill cause the switch arm 65 to separate from the hot Contact 66breaking the running circuit for the burner motor. After the startingcircuit has been reclosed by the bellows 12 at the switch contacts19-83, the system will recycle.

It will therefore be seen that I have designed a relatively simple andinexpensive oil burner control systeminvolving the use of noelectromagnetic relays but yet one which is rendered safe in the eventof a momentary power failure through the use of a fuel pressureresponsive switch in the running circuit to the burner motor. It isobvious that certain changes and modifications may be made in thedetails of the two systems disclosed without the use of invention, and Itherefore desire it to be understood that I intend to be limited only bythe scope of the appended claims.

I claim as my invention:

1. A control system of the class described comprising in combination, aburner motor for delivering fuel under pressure at a point ofcombustion, a starting circuit and a running circuit for said motor, aswitch in said running circuit, means responsive to the pressureproduced by said motor for closing said switch, a switch operated by thepressure produced by said motor for opening said starting circuit, andmeans for delaying the closure of said switch for a sufficient timefollowing the failure of said pressure to provide for a proper scavengerperiod.

2. A control system of the class described comprising in combination, aburner motor for delivering fuel under pressure at a point ofcombustion, a starting circuit and a running circuit for said motor, aswitch in said running circuit, means responsive to the pressureproduced by said motor for closing said switch, a second switch in saidrunning circuit, means responsive to the establishment of combustion forclosing said second switch, a switch operated by the pressure developedby said motor for opening said starting circuit a predetermined timeafter its closure, and means for delaying the closure of said switch fora suicient time following the failure of said pressure to provide for aproper scavenger period.

3. A control system of the class described comprising in combination, aburner motor for delivering fuel under pressure at a point ofcombustion, a starting circuit and a running circuit for said motor, aswitch in said running circuit, means responsive to the pressureproduced by said motor for closing said switch, a time delay switch foropening said starting circuit a predetermined time after its closure,and means for adjusting the timing of said time delay switch to providefor a proper scavenger period.

4. A control system of the class described comprising in combination, aburner motor for delivering fuel under pressure at a point ofcombustion, a starting circuit and a running circuit for said motor, aswitch in said running circuit, means responsive to the pressureproduced by said motor for closing said switch, a time delay switch foropening said starting circuit a predetermined time after its closure,said time delay switch being also operated by the pressure produced bysaid motor, and means for adjusting the timing of said time delay switchto provide for a proper scavenger period.

5. A control system of the class described comprising in combination, aburner motor for delivering fuel under pressure at a point ofcombustion, a starting circuit and a running circuit for said motor, amain control switch and a normally closed thermally actuated safetyswitch in control of both circuits, a heater for said safety switch insaid starting circuit, a. normally closed switch in said startingcircuit, time delay means responsive to the pressure developed by saidmotor for opening said last named switch after a predetermined timeinterval, said time interval being greater than the timing of saidsafety switch, a switch in said running circuit closed by the pressuredeveloped by said motor, and a second switch in said running circuitclosed in response to the establishment of combustion.

6. A control system of the class described comprising in combination, aburner motor for delivering fuel under pressure at a point ofcombustion, a starting circuit and a running circuit for said motor, amain control switch and a normally closed thermally actuated safetyswitch in control of both circuits, a heater for said safetyswitclLirLsaiJdstai-ting circuit, a normally closed switch in saids`t``ft`ig circuit, time delay means responsive to the pressuredeveloped by said motor for opening said last named switch after apredetermined time interval, said time interval being greater than thetiming of said safety switch, a switch in said running circuit closed bythe pressure developed by said motor, a second switch in said runningcircuit closed in response to the establishment of combustion, said timedelay means introducing another time delay between the deenergization ofsaid motor and the reclosing of said normally closed switch, and meansfor adjusting the timing of said time delay means.

7. A control system of the class described comprising in combination, aburner motor for delivering fuel under pressure at a point ofcombustion, a starting circuit and a running circuit for said motor, anormally closed switch in said starting circuit, a normally open switchin said running circuit, a chamber in which said motor, when energized,builds up a pressure, pressure responsive means in restrictedcommunication with said chamber, lost motion means connecting saidpressure responsive means to said normally closed switch for opening andclosing said switch a predetermined time after the energization anddeenergization of said motor, and slip friction means connecting saidpressure responsive means to said normally open switch whereby thelatter is closed and opened substantially immediately upon energizationand deenergization of said motor.

8. A control system of the class described comprising in combination, aburner motor for delivering fuel under pressure at a point ofcombustion, a starting circuit and a running circuit for said motor, anormally closed switch in said starting circuit, a normally open switchin said running circuit, a chamber in which said motor, when energized,builds up a pressure, pressure responsive means in restrictedcommunication with said chamber, lost motion means connecting saidpressure responsive means to said normally closed switch for opening andclosing said switch a predetermined time after the energization anddeenergization of said motor, slip friction means connecting saidpressure responsive means to said normally open switch whereby thelatter is closed and opened substantially immediately upon energizationand deenergization of said motor, and means for adjusting the degree ofrestriction to vary the timing of operation of said normally closedswitch.

WILLIAM L. MCGRATH.

